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<h2><span class="refentrytitle"><a name="glib-Warnings-and-Assertions.top_of_page"></a>Message Output and Debugging Functions</span></h2>
<p>Message Output and Debugging Functions — functions to output messages and help debug applications</p>
</td>
<td valign="top" align="right"></td>
</tr></table></div>
<div class="refsynopsisdiv">
<a name="glib-Warnings-and-Assertions.synopsis"></a><h2>Synopsis</h2>
<pre class="synopsis">
#include &lt;glib.h&gt;

<span class="returnvalue">void</span>                <a class="link" href="glib-Warnings-and-Assertions.html#g-print" title="g_print ()">g_print</a>                             (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *format</code></em>,
                                                         <em class="parameter"><code>...</code></em>);
<a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="returnvalue">GPrintFunc</span></a>          <a class="link" href="glib-Warnings-and-Assertions.html#g-set-print-handler" title="g_set_print_handler ()">g_set_print_handler</a>                 (<em class="parameter"><code><a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="type">GPrintFunc</span></a> func</code></em>);
<span class="returnvalue">void</span>                (<a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()">*GPrintFunc</a>)                       (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *string</code></em>);

<span class="returnvalue">void</span>                <a class="link" href="glib-Warnings-and-Assertions.html#g-printerr" title="g_printerr ()">g_printerr</a>                          (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *format</code></em>,
                                                         <em class="parameter"><code>...</code></em>);
<a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="returnvalue">GPrintFunc</span></a>          <a class="link" href="glib-Warnings-and-Assertions.html#g-set-printerr-handler" title="g_set_printerr_handler ()">g_set_printerr_handler</a>              (<em class="parameter"><code><a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="type">GPrintFunc</span></a> func</code></em>);

#define             <a class="link" href="glib-Warnings-and-Assertions.html#g-return-if-fail" title="g_return_if_fail()">g_return_if_fail</a>                    (expr)
#define             <a class="link" href="glib-Warnings-and-Assertions.html#g-return-val-if-fail" title="g_return_val_if_fail()">g_return_val_if_fail</a>                (expr,
                                                         val)
#define             <a class="link" href="glib-Warnings-and-Assertions.html#g-return-if-reached" title="g_return_if_reached">g_return_if_reached</a>
#define             <a class="link" href="glib-Warnings-and-Assertions.html#g-return-val-if-reached" title="g_return_val_if_reached()">g_return_val_if_reached</a>             (val)
#define             <a class="link" href="glib-Warnings-and-Assertions.html#g-warn-if-fail" title="g_warn_if_fail()">g_warn_if_fail</a>                      (expr)
#define             <a class="link" href="glib-Warnings-and-Assertions.html#g-warn-if-reached" title="g_warn_if_reached">g_warn_if_reached</a>

<span class="returnvalue">void</span>                <a class="link" href="glib-Warnings-and-Assertions.html#g-on-error-query" title="g_on_error_query ()">g_on_error_query</a>                    (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *prg_name</code></em>);
<span class="returnvalue">void</span>                <a class="link" href="glib-Warnings-and-Assertions.html#g-on-error-stack-trace" title="g_on_error_stack_trace ()">g_on_error_stack_trace</a>              (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *prg_name</code></em>);

#define             <a class="link" href="glib-Warnings-and-Assertions.html#G-BREAKPOINT:CAPS" title="G_BREAKPOINT">G_BREAKPOINT</a>
</pre>
</div>
<div class="refsect1">
<a name="glib-Warnings-and-Assertions.description"></a><h2>Description</h2>
<p>
These functions provide support for outputting messages.
</p>
<p>
The <code class="function">g_return</code> family of macros (<a class="link" href="glib-Warnings-and-Assertions.html#g-return-if-fail" title="g_return_if_fail()"><code class="function">g_return_if_fail()</code></a>,
<a class="link" href="glib-Warnings-and-Assertions.html#g-return-val-if-fail" title="g_return_val_if_fail()"><code class="function">g_return_val_if_fail()</code></a>, <a class="link" href="glib-Warnings-and-Assertions.html#g-return-if-reached" title="g_return_if_reached"><code class="function">g_return_if_reached()</code></a>, <a class="link" href="glib-Warnings-and-Assertions.html#g-return-val-if-reached" title="g_return_val_if_reached()"><code class="function">g_return_val_if_reached()</code></a>)
should only be used for programming errors, a typical use case is
checking for invalid parameters at the beginning of a public function.
They should not be used if you just mean "if (error) return", they
should only be used if you mean "if (bug in program) return".
The program behavior is generally considered undefined after one of these
checks fails. They are not intended for normal control flow, only to
give a perhaps-helpful warning before giving up.
</p>
</div>
<div class="refsect1">
<a name="glib-Warnings-and-Assertions.details"></a><h2>Details</h2>
<div class="refsect2">
<a name="g-print"></a><h3>g_print ()</h3>
<pre class="programlisting"><span class="returnvalue">void</span>                g_print                             (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *format</code></em>,
                                                         <em class="parameter"><code>...</code></em>);</pre>
<p>
Outputs a formatted message via the print handler.
The default print handler simply outputs the message to stdout.
</p>
<p>
<a class="link" href="glib-Warnings-and-Assertions.html#g-print" title="g_print ()"><code class="function">g_print()</code></a> should not be used from within libraries for debugging messages,
since it may be redirected by applications to special purpose message
windows or even files.
Instead, libraries should use <a class="link" href="glib-Message-Logging.html#g-log" title="g_log ()"><code class="function">g_log()</code></a>, or the convenience functions
<a class="link" href="glib-Message-Logging.html#g-message" title="g_message()"><code class="function">g_message()</code></a>, <a class="link" href="glib-Message-Logging.html#g-warning" title="g_warning()"><code class="function">g_warning()</code></a> and <a class="link" href="glib-Message-Logging.html#g-error" title="g_error()"><code class="function">g_error()</code></a>.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>format</code></em> :</span></p></td>
<td>the message format. See the <code class="function">printf()</code> documentation.</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td>
<td>the parameters to insert into the format string.</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-set-print-handler"></a><h3>g_set_print_handler ()</h3>
<pre class="programlisting"><a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="returnvalue">GPrintFunc</span></a>          g_set_print_handler                 (<em class="parameter"><code><a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="type">GPrintFunc</span></a> func</code></em>);</pre>
<p>
Sets the print handler.
Any messages passed to <a class="link" href="glib-Warnings-and-Assertions.html#g-print" title="g_print ()"><code class="function">g_print()</code></a> will be output via the new handler.
The default handler simply outputs the message to stdout.
By providing your own handler you can redirect the output, to a GTK+
widget or a log file for example.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>func</code></em> :</span></p></td>
<td>the new print handler.</td>
</tr>
<tr>
<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td>
<td>the old print handler.</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="GPrintFunc"></a><h3>GPrintFunc ()</h3>
<pre class="programlisting"><span class="returnvalue">void</span>                (*GPrintFunc)                       (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *string</code></em>);</pre>
<p>
Specifies the type of the print handler functions.
These are called with the complete formatted string to output.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody><tr>
<td><p><span class="term"><em class="parameter"><code>string</code></em> :</span></p></td>
<td>the message to be output.</td>
</tr></tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-printerr"></a><h3>g_printerr ()</h3>
<pre class="programlisting"><span class="returnvalue">void</span>                g_printerr                          (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *format</code></em>,
                                                         <em class="parameter"><code>...</code></em>);</pre>
<p>
Outputs a formatted message via the error message handler.
The default handler simply outputs the message to stderr.
</p>
<p>
<a class="link" href="glib-Warnings-and-Assertions.html#g-printerr" title="g_printerr ()"><code class="function">g_printerr()</code></a> should not be used from within libraries. Instead <a class="link" href="glib-Message-Logging.html#g-log" title="g_log ()"><code class="function">g_log()</code></a> should
be used, or the convenience functions <a class="link" href="glib-Message-Logging.html#g-message" title="g_message()"><code class="function">g_message()</code></a>, <a class="link" href="glib-Message-Logging.html#g-warning" title="g_warning()"><code class="function">g_warning()</code></a> and <a class="link" href="glib-Message-Logging.html#g-error" title="g_error()"><code class="function">g_error()</code></a>.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>format</code></em> :</span></p></td>
<td>the message format. See the <code class="function">printf()</code> documentation.</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td>
<td>the parameters to insert into the format string.</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-set-printerr-handler"></a><h3>g_set_printerr_handler ()</h3>
<pre class="programlisting"><a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="returnvalue">GPrintFunc</span></a>          g_set_printerr_handler              (<em class="parameter"><code><a class="link" href="glib-Warnings-and-Assertions.html#GPrintFunc" title="GPrintFunc ()"><span class="type">GPrintFunc</span></a> func</code></em>);</pre>
<p>
Sets the handler for printing error messages.
Any messages passed to <a class="link" href="glib-Warnings-and-Assertions.html#g-printerr" title="g_printerr ()"><code class="function">g_printerr()</code></a> will be output via the new handler.
The default handler simply outputs the message to stderr.
By providing your own handler you can redirect the output, to a GTK+
widget or a log file for example.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>func</code></em> :</span></p></td>
<td>the new error message handler.</td>
</tr>
<tr>
<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td>
<td>the old error message handler.</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-return-if-fail"></a><h3>g_return_if_fail()</h3>
<pre class="programlisting">#define             g_return_if_fail(expr)</pre>
<p>
Returns from the current function if the expression is not true.
If the expression evaluates to <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a>, a critical message is logged and
the function returns. This can only be used in functions which do not return
a value.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody><tr>
<td><p><span class="term"><em class="parameter"><code>expr</code></em> :</span></p></td>
<td>the expression to check.</td>
</tr></tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-return-val-if-fail"></a><h3>g_return_val_if_fail()</h3>
<pre class="programlisting">#define             g_return_val_if_fail(expr,val)</pre>
<p>
Returns from the current function, returning the value <em class="parameter"><code>val</code></em>, if the expression
is not true.
If the expression evaluates to <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a>, a critical message is logged and
<em class="parameter"><code>val</code></em> is returned.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>expr</code></em> :</span></p></td>
<td>the expression to check.</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>val</code></em> :</span></p></td>
<td>the value to return from the current function if the expression is not
true.</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-return-if-reached"></a><h3>g_return_if_reached</h3>
<pre class="programlisting">#define             g_return_if_reached()</pre>
<p>
Logs a critical message and returns from the current function.
This can only be used in functions which do not return a value.
</p>
</div>
<hr>
<div class="refsect2">
<a name="g-return-val-if-reached"></a><h3>g_return_val_if_reached()</h3>
<pre class="programlisting">#define             g_return_val_if_reached(val)</pre>
<p>
Logs a critical message and returns <em class="parameter"><code>val</code></em>.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody><tr>
<td><p><span class="term"><em class="parameter"><code>val</code></em> :</span></p></td>
<td>the value to return from the current function.</td>
</tr></tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="g-warn-if-fail"></a><h3>g_warn_if_fail()</h3>
<pre class="programlisting">#define             g_warn_if_fail(expr)</pre>
<p>
Logs a warning if the expression is not true.
</p>
<div class="variablelist"><table border="0">
<col align="left" valign="top">
<tbody><tr>
<td><p><span class="term"><em class="parameter"><code>expr</code></em> :</span></p></td>
<td>the expression to check</td>
</tr></tbody>
</table></div>
<p class="since">Since 2.16</p>
</div>
<hr>
<div class="refsect2">
<a name="g-warn-if-reached"></a><h3>g_warn_if_reached</h3>
<pre class="programlisting">#define             g_warn_if_reached()</pre>
<p>
Logs a critical warning.
</p>
<p class="since">Since 2.16</p>
</div>
<hr>
<div class="refsect2">
<a name="g-on-error-query"></a><h3>g_on_error_query ()</h3>
<pre class="programlisting"><span class="returnvalue">void</span>                g_on_error_query                    (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *prg_name</code></em>);</pre>
<p>
Prompts the user with <code class="computeroutput">[E]xit, [H]alt, show [S]tack trace or [P]roceed</code>.
This function is intended to be used for debugging use only. The following
example shows how it can be used together with the <a class="link" href="glib-Message-Logging.html#g-log" title="g_log ()"><code class="function">g_log()</code></a> functions.
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3
4
5
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9
10
11
12
13
14
15
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18
19</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="preproc">#include</span><span class="normal"> </span><span class="string">&lt;glib.h&gt;</span>
<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">log_handler</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">gchar</span><span class="normal">   </span><span class="symbol">*</span><span class="normal">log_domain</span><span class="symbol">,</span>
<span class="normal">         </span><span class="usertype">GLogLevelFlags</span><span class="normal"> log_level</span><span class="symbol">,</span>
<span class="normal">         </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">gchar</span><span class="normal">   </span><span class="symbol">*</span><span class="normal">message</span><span class="symbol">,</span>
<span class="normal">         </span><span class="usertype">gpointer</span><span class="normal">       user_data</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="function"><a href="glib-Message-Logging.html#g-log-default-handler">g_log_default_handler</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">log_domain</span><span class="symbol">,</span><span class="normal"> log_level</span><span class="symbol">,</span><span class="normal"> message</span><span class="symbol">,</span><span class="normal"> user_data</span><span class="symbol">);</span>
<span class="normal">  </span><span class="function"><a href="glib-Warnings-and-Assertions.html#g-on-error-query">g_on_error_query</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">MY_PROGRAM_NAME</span><span class="symbol">);</span>
<span class="cbracket">}</span>
<span class="type">int</span><span class="normal"> </span><span class="function">main</span><span class="normal"> </span><span class="symbol">(</span><span class="type">int</span><span class="normal"> argc</span><span class="symbol">,</span><span class="normal"> </span><span class="type">char</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">argv</span><span class="symbol">[])</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="function"><a href="glib-Message-Logging.html#g-log-set-handler">g_log_set_handler</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">MY_LOG_DOMAIN</span><span class="symbol">,</span>
<span class="normal">             <a href="glib-Message-Logging.html#G-LOG-LEVEL-WARNING:CAPS">G_LOG_LEVEL_WARNING</a> </span><span class="symbol">|</span>
<span class="normal">                     <a href="glib-Message-Logging.html#G-LOG-LEVEL-ERROR:CAPS">G_LOG_LEVEL_ERROR</a> </span><span class="symbol">|</span>
<span class="normal">                     <a href="glib-Message-Logging.html#G-LOG-LEVEL-CRITICAL:CAPS">G_LOG_LEVEL_CRITICAL</a></span><span class="symbol">,</span>
<span class="normal">             log_handler</span><span class="symbol">,</span>
<span class="normal">             <a href="glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="comment">/* ... */</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p>
If [E]xit is selected, the application terminates with a call to
<code class="function">_exit(0)</code>.
</p>
<p>
If [H]alt is selected, the application enters an infinite loop.
The infinite loop can only be stopped by killing the application,
or by setting <span class="type">glib_on_error_halt</span> to <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a> (possibly via a debugger).
</p>
<p>
If [S]tack trace is selected, <a class="link" href="glib-Warnings-and-Assertions.html#g-on-error-stack-trace" title="g_on_error_stack_trace ()"><code class="function">g_on_error_stack_trace()</code></a> is called. This
invokes <span class="command"><strong>gdb</strong></span>, which attaches to the current process and shows a stack trace.
The prompt is then shown again.
</p>
<p>
If [P]roceed is selected, the function returns.
</p>
<p>
This function may cause different actions on non-UNIX platforms.
</p>
<div class="variablelist"><table border="0">
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<td><p><span class="term"><em class="parameter"><code>prg_name</code></em> :</span></p></td>
<td>the program name, needed by <span class="command"><strong>gdb</strong></span> for the [S]tack trace option.
If <em class="parameter"><code>prg_name</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, <a class="link" href="glib-Miscellaneous-Utility-Functions.html#g-get-prgname" title="g_get_prgname ()"><code class="function">g_get_prgname()</code></a> is called to get the program name
(which will work correctly if <a href="http://library.gnome.org/devel/gdk3/gdk-General.html#gdk-init"><code class="function">gdk_init()</code></a> or <a href="http://library.gnome.org/devel/gtk/gtk3-General.html#gtk-init"><code class="function">gtk_init()</code></a> has been called).</td>
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<div class="refsect2">
<a name="g-on-error-stack-trace"></a><h3>g_on_error_stack_trace ()</h3>
<pre class="programlisting"><span class="returnvalue">void</span>                g_on_error_stack_trace              (<em class="parameter"><code>const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar"><span class="type">gchar</span></a> *prg_name</code></em>);</pre>
<p>
Invokes <span class="command"><strong>gdb</strong></span>, which attaches to the current process and shows a stack trace.
Called by <a class="link" href="glib-Warnings-and-Assertions.html#g-on-error-query" title="g_on_error_query ()"><code class="function">g_on_error_query()</code></a> when the [S]tack trace option is selected.
</p>
<p>
This function may cause different actions on non-UNIX platforms.
</p>
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<td><p><span class="term"><em class="parameter"><code>prg_name</code></em> :</span></p></td>
<td>the program name, needed by <span class="command"><strong>gdb</strong></span> for the [S]tack trace option.
If <em class="parameter"><code>prg_name</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, <a class="link" href="glib-Miscellaneous-Utility-Functions.html#g-get-prgname" title="g_get_prgname ()"><code class="function">g_get_prgname()</code></a> is called to get the program name
(which will work correctly if <a href="http://library.gnome.org/devel/gdk3/gdk-General.html#gdk-init"><code class="function">gdk_init()</code></a> or <a href="http://library.gnome.org/devel/gtk/gtk3-General.html#gtk-init"><code class="function">gtk_init()</code></a> has been called).</td>
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<div class="refsect2">
<a name="G-BREAKPOINT:CAPS"></a><h3>G_BREAKPOINT</h3>
<pre class="programlisting">#  define G_BREAKPOINT()        G_STMT_START{ __asm__ __volatile__ ("int $03"); }G_STMT_END
</pre>
<p>
Inserts a breakpoint instruction into the code.  On x86 and alpha systems
this is implemented as a soft interrupt and on other architectures it raises
a <code class="literal">SIGTRAP</code> signal.
</p>
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